The BCL-2 family constitutes a crucial checkpoint in the cell death pathway. BID is a member of the "BH3-only" subset of this family and requires its conserved BH3 domain for its pro-apoptotic function. BID plays an important role in the apoptotic pathway downstream of death receptors such as TNF RI and Fas (1-7). Cleavage by caspase 8 and myristoylation serves as an activating molecular switch which facilitates targeting of BID to mitochondria(8) where it activates multidomain proapoptotic members such as BAK or BAX, mediating the release of cytochrome c and activation of downstream effector caspases (9). Bid-deficient mice are viable and develop normally, however hepatocytes are resistant to anti-Fas-induced hepatocellular apoptosis(10). BID thus plays an important role in mediating a mitochondrial amplification loop downstream of death receptors in certain cell types such as hepatocytes. Yet many other cells, including activated lymphocytes, are killed by Fas in in vitro assays, directly activating downstream effector caspases for the promotion of apoptosis(11). However, as Bid-deficient mice age or are challenged by chronic viral infection, we've discovered that BID is also singularly required in vivo to maintain homeostasis in so-called Type I cells including B cells, T cells, and the myelomonocytic lineage(12).As Bid-deficient mice age, they display a myeloid hyperplasia and then progress to Chronic myelomonocytic leukemia at high incidence. The founding family member, BCL-2 was cloned at the t(14:18) chromosomal breakpoint, the molecular hallmark of follicular B cell lymphoma establishing a new class of oncogenes in which the aberration is in cell death rather than proliferation(13-15). Transgenic mice over expressing BCL-2 in myeloid cells develop a myeloproliferative disorder, and when crossed with mice harboring a mutation in the Fas receptor, progress to AML, implicating a synergistic role for the Fas pathway and Bcl-2 in tumor suppression of the myeloid lineage(16). The role ofpro- apoptotic members in normal myeloid homeostasis has not been well characterized. We propose that BID is the critical proapoptotic intermediate that mediates death receptor signalling in the myeloid lineage and that its inactivations promotes myeloid leukemogenesis by prolonging the life of these vulnerable cells, allowing accumulation of additional mutations. This single "BH3- only" protein therefore has potential to play a potent role in maintenance of normal myeloid homeostasis as well as tumor suppression. In this context, I propose the following specific aims: 1. Define the role of Bid in myeloid homeostasis. 2. Define the role of Bid in myeloid leukemogenesis. 3. Identify the secondary genetic changes that cooperate with loss of Bid in leukemogenesis.